The present invention relates to a field distribution converting optical fiber, and a laser diode module using the field distribution converting optical fiber, which are used when connecting optical components having different field distributions to each other in a case where, for example, in an optical transmission and sensor field, an optical component having a circular cross section to an optical component having an elliptical cross section exists.
Optical fibers have been used for optical transmission, and the cores of optical fibers are covered with cladding. The cross sectional shape of the core of an optical fiber generally is circular (that is, the section where an optical fiber is cut off in a plane XY where it is assumed that the optical axis of the optical fiber is Z, and three axes orthogonal to each other are X, Y and Z). In addition, the light intensity distribution, on the plane XY, of light propagating in the core is also circular. On the other hand, as for a semiconductor laser and a planar optical waveguide, generally, the light intensity distribution on the plane XY is elliptical. Therefore, when connecting an optical component of a semiconductor laser and a planar optical waveguide, etc., to the above optical fiber, an optical connection loss occurs due to a difference between the field distribution (electric field distribution) at the connection end faces of these optical components and the field distribution of the optical fiber. Moreover, an optical connection loss resulting from such a difference between the field distributions is called a mode non-matching loss.
For example, FIG. 13 shows a result of an investigation of an optical connection loss between an optical component having an elliptical field distribution and an optical component having a circular field distribution, changing the aspect ratio (ratio of the major axis to the minor axis) of an elliptical shape in the optical component having an elliptical field distribution. As has been made clear in this drawing, if an optical component having elliptical field distribution exists, the aspect ratio of which is 3 or more, to an optical component having circular field distribution, it can be understood that an optical connection loss of 2dB or more occurs. In addition, in FIG. 13, the spot diameter of either the X or Y direction of the above elliptical field distribution is made coincident with the spot diameter of the above circular field distribution.
Therefore, for example, the following optical fiber is disclosed in Laid-open patent application No. 33706 of 1988 filed and laid open in Japan. That is, the optical fiber has an elliptical core 2 such as that shown in FIG. 14, wherein one end side of a single mode optical fiber, in which the refractive index distribution of the core 2 is uniform, is heated to cause a dopant to be diffused, which is contained in the core 2 or cladding 3, whereby the cross-sectional shape of the core 2 at one end side of the optical fiber is made roughly circular. Then, the core profile becomes an elliptical shape at the end side 11 of an optical fiber, and the core profile at the end side 12 (where the above heating is carried out) becomes a circular shape. In such an optical fiber, the field distribution of optical waves propagating in the optical fiber becomes elliptical at the end side 11 of the optical fiber, and becomes circular at the end side 12 of the optical fiber, wherein mutual conversion of an ellipse and a circle in the field distribution is performed, and such an optical fiber can be used as a field distribution converting optical fiber.
However, in a field distribution converting optical fiber formed by utilizing dopant diffusion like an optical fiber shown in FIG. 14, there were the following shortcomings and problems.
(1) In the optical fiber shown in FIG. 14, the normalizing frequency viewed in the greater diameter direction of an elliptical core before diffusion of a dopant is different from that view in the minor diameter direction, and the aspect ratio of the field distribution does not become greater than the aspect ratio of the core 2. Therefore, the aspect ratio of the field distribution of the end part 11 of an optical fiber is smaller than the aspect ratio of the core 2, for example, the aspect ratio of the field distribution is 2 at the most, wherein such an elliptical optical fiber will have a field distribution very similar to that of a circular optical fiber. Therefore, if such an optical fiber as shown in FIG. 14 is used, mutual conversion is impossible between an ellipse and a circle as regards the field distribution.
(2) If the above dopant diffusion is utilized, the elliptical field distribution may be shaped to be circular in only the direction of expanding the field distribution. Therefore, there was such a problem by which a large elliptical profile could not be converted to a small circular profile.
(3) Since it takes a long time to obtain a dopant diffusion, productivity is not satisfactory. Moreover, unless the portion at which a part of an. optical fiber is partially heated is accurately taken out, it is impossible to utilize the optical fiber as a field distribution converting optical fiber. Therefore, after being partially heated, the optical fiber must be accurately cut off one by one, whereby working efficiency worsens, and it is difficult to mass produce the field distribution converting optical fibers.
It is therefore an object of the invention to provide a field distribution converting optical fiber which can solve such shortcomings and problems, and a laser diode module using the field distribution converting optical fiber.
That is, it is an object of the invention to provide a field distribution converting optical fiber, which can be mass produced, capable of universally carrying out mutual conversion between field distribution of an elliptical profile and field distribution of a circular profile in either direction of reduction, equal magnification (mutual conversion between a circle having a diameter which is the same as the major or minor axis of an ellipse, and an ellipse), or enlargement as, for example, in conversion between the field distribution of a laser diode and the field distribution of an optical fiber, and a laser diode module using the field distribution converting optical fiber.
A field distribution converting optical fiber according to the first aspect of the invention is an optical fiber having cladding covered around its core, which is composed so that the optical axis is assumed to be Z, either of the refractive index profiles of the above core in the X and Y directions of the orthogonal three axes X, Y and Z orthogonal to each other is made into a square-law distribution profile, a slope of the refractive index distribution in the corresponding X direction is made different from a slope of the refractive index distribution in the Y direction, and the optical fiber length is made almost coincident with the length of the common multiple between a one-fourth cycle length of a field distribution change cycle of a light beam propagating in the XZ plane in the Z direction and a one-fourth cycle length of a field distribution change cycle of a light beam propagating in the YZ plane in the Z direction.
A field distribution converting optical fiber according to the second aspect of the invention is an optical fiber having cladding covered around its core, which is composed so that the optical axis is assumed to be Z, either of the refractive index profiles of the above core in the X and Y directions of the orthogonal three axes X, Y and Z orthogonal to each other is made into a square-law distribution profile, a slope of the refractive index distribution in the corresponding X direction is made different from a slope of the refractive index distribution in the Y direction, either field distribution in the X direction or Y direction satisfies conditions for which light does not change while propagating in the optical fiber, and the optical fiber length is almost coincident with the length of a multiple of a one-fourth cycle length of the field distribution change cycle, in the Z direction, of a light beam propagating in the XZ plane or YZ plane, which changes due to propagation of light.
A field distribution converting optical fiber according to the third aspect of the invention is characterized in that, in a field distribution converting optical fiber provided with the first and second aspects, the cross-sectional shape of the core is made roughly elliptical, roughly ellipsoidal-shaped or roughly rectangular.
A field distribution converting optical fiber according to the fourth aspect of the invention is characterized in that a single mode optical fiber is provided at one end side of a field distribution converting optical fiber according to the first or second aspect of the invention.
A field distribution converting optical fiber according to the fifth aspect of the invention is characterized in that a single mode optical fiber is provided at one end side of a field distribution converting optical fiber according to the third aspect of the invention.
A field distribution converting optical fiber according to the sixth aspect of the invention is an optical fiber having cladding covered around the core, and is characterized in that the optical axis is assumed to be Z, either of the refractive index profiles of the above core in the X and Y directions of the orthogonal three axes X, Y and Z orthogonal to each other is made into a square-law distribution profile, the slope of the refractive index distribution in the corresponding X direction is made equal to the slope of the refractive index distribution in the Y direction, wherein an optical fiber is formed so that the optical fiber length thereof is made almost coincident with the odd number times the one-fourth cycle of the field distribution changing cycle, in the Z axis direction, of a light beam propagating in the XZ plane and YZ plane, and the optical fiber intervenes between a field distribution converting optical fiber according to the first or second aspect and the single mode optical fiber.
A field distribution converting optical fiber according to the seventh aspect of the invention is an optical fiber having cladding covered around the core, and is characterized in that the optical axis is assumed to be Z, either of the refractive index profiles of the above core in the X and Y directions of the orthogonal three axes X, Y and Z orthogonal to each other is made into a square-law distribution profile, the slope of the refractive index distribution in the corresponding X direction is made equal to the slope of the refractive index distribution in the Y direction, wherein an optical fiber is formed so that the optical fiber length thereof is made almost coincident with the odd number times the one-fourth cycle of the field distribution changing cycle, in the Z axis direction, of a light beam propagating in the XZ plane and YZ plane, and the optical fiber intervenes between a field distribution converting optical fiber according to the third aspect and the single mode optical fiber.
A laser diode module in which a field distribution converting optical fiber according to the invention is used is characterized in that a laser diode is connected to the input side of a field distribution converting fiber as set forth in any one of the first to seventh aspects of the invention.
It is commonly known that as regards a field distribution of optical waves propagating in an optical fiber in which the refractive index profiles of a core in the X and Y directions are square-law distribution profiles, the spot diameter thereof cyclically changes when the optical axis of the optical fiber is Z and the three axes orthogonal to each other are X, Y and Z.
Therefore, the present applicant, et al. propagated Gaussian type light, that is, light of field distribution in which the optical intensity distribution becomes normal distribution with respect to the radius of an optical fiber, in an optical fiber of a circular core in which the refractive index profile in the above X, Y directions is a square-law distribution profile. And, the applicant, et al. investigated how the field distribution of light changed in the lengthwise direction of the optical fiber when the light was propagated in the optical fiber.
The result is shown in FIG. 12. Further, in the same drawing, the abscissa indicates a propagation distance of optical waves, and the ordinate indicates the spot diameter of field distribution. The field distribution of optical waves propagating in optical fibers has a tendency where it becomes greater and greater toward the center of the optical fibers and becomes smaller and smaller toward the outer circumference of the optical fibers. For example, FIG. 5(b) shows one example of a field distribution of optical waves incident into the optical. fibers in the Y direction on the XY sectional plane. Therefore, in the present specification, it is assumed that, as shown in FIG. 5(b), the width of a range, which has a greater field size than 1/e (where e is the bottom of the natural logarithm) where the center field distribution is 1, is called a xe2x80x9cspot diameterxe2x80x9d.
In addition, where the field distribution size is expressed in terms of intensity, for example, where the center field intensity is 1, a range having field intensity distribution which is greater than the intensity of 1/e2 is called a spot diameter.
As has been made clear in FIG. 12, the spot diameter of field distribution cyclically changes in the lengthwise direction of an optical fiber in line with propagation of light. In detail, in the optical fiber, the above spot diameter which is 10 xcexcm at the incident end of the optical fiber cyclically changes in a range from 10 xcexcm to 6 xcexcm in line with propagation of light.
Furthermore, the example illustrated in FIG. 12 shows the results obtained with respect to an optical fiber, the Ag value of which is 45 xcexcm where Ag=a/{square root over ((2+L xcex4))} is defined when the relative refractive index difference (n1-n2)/n2 between the refractive index n1 of the core center of an optical fiber and the refractive index n2 of cladding is xcex94, and the core radius is a. Resultantly, when propagating optical waves are expressed in terms of light beam, the field distribution change cycle of the light beam in the Z axis direction becomes 2xcfx80Ag, and it is found that the spot diameter of field distribution repeats the minimum value and the maximum value once every one-fourth cycle length of the field distribution change cycle of the light beam.
Also, where the spot radii (half the spot diameter) at adjacent extreme values, respectively, were w1 and w2 and the wavelength of light was xcex, it was found on the basis of examinations carried out by the applicant that the relationship of w1xc2x7w2=xcexAg/nxcfx80 is established. It was also found that, at a position which is the integral number times the one-fourth cycle length of the above field distribution change cycle, the phase distribution of light beam became flat. In other words, it was found that light could reach the position, which is the integral number times the one-fourth cycle length of the above field distribution change cycle, in a state where the speeds of light beams passing through various channels in an optical fiber are made coincident with each other and the light intensity of light was not produced due to the interference of light passing through the above various channels.
Based on the results of the above examinations, in the field distribution converting optical fiber thus constructed according to the invention, the field distribution of optical waves is designed so as to cyclically change with respect to the lengthwise direction (Z axis direction) of an optical fiber, as has been made clear in the results of the examinations shown in FIG. 12, by making either of the refractive index profiles of a core of three axes X, Y and Z orthogonal to each other in the X or Y directions into a square-law distribution profile as shown in, for example, FIGS. 11(a) and (b), where the optical axis of the optical fiber is Z.
Further, in a field distribution converting optical fiber according to the invention, as shown in FIG. 11, by making slopes different from each other where either of the refractive index distributions in the above X or Y direction is made into a square-law distribution profile, the field distribution changing cycle of light beams propagating in the XZ plane and the field distribution changing cycle of light beams propagating in the YZ plane differ in the X and Y directions. Therefore, by making the slopes of the square-law distribution profile into values which are different from each other, the field distribution changing cycle of light beams propagating in the XZ plane and the field distribution changing cycle of light beams propagating in the YZ plane are made different in the X and Y directions, wherein the cross-sectional profile (the sectional profile where being cut off in the XY plane) of the field distribution with respect to a propagation distance of optical waves propagating in an optical fiber can be cyclically made elliptical or circular. In addition, in a field distribution converting optical fiber according to the invention, as described above, by making either of the refractive index distributions in the above X or Y direction into a square-law distribution profile and making the slope thereof into values which are different from each other, the aspect ratio of an ellipse can be cyclically changed freely.
With a field distribution converting optical fiber according to the first aspect of the invention, the optical axis of an optical fiber is made into Z, the refractive index profiles of the core in the X and Y directions of the three axes X, Y, and Z orthogonal to each other are made into a square-law distribution profile in either case, the field distribution of optical waves is designed so as to cyclically change with respect to the lengthwise direction (Z axis direction) of the optical fiber, and slopes of the refractive index distribution in the above X and Y directions are made into values which are different from each other, wherein the cycle of field distribution change of a light beam propagating in the XY plane can be made different from the cycle of field distribution change of a light beam propagating in the XZ plane. Therefore, with a field distribution converting optical fiber according to the invention, the cross-sectional profile (the sectional profile where being cut off in the XY plane) of a field distribution with respect to a propagation distance of optical waves propagating in an optical fiber can be cyclically made circular or elliptical. Further, the aspect ratio of an ellipse can be cyclically changed freely.
And, with a field distribution converting optical fiber according to the first aspect of the invention, since an optical fiber length is constructed so as to be made coincident with a length which is a common multiple of the one-fourth cycle length of a field distribution change cycle, in the Z axis direction, of a light beam propagating in the XZ plane, and a one-fourth cycle length of a field distribution change cycle, in the Z axis direction, of a light beam propagating in the YZ plane, for example, optical waves having field distributions of an elliptical profile are made incident from the incident side of an optical fiber and the field distribution at the outgoing side of the optical fiber can be made circular, and the reverse can be carried out. Further, with a field distribution converting optical fiber according to the first aspect of the invention, mutual conversion from elliptical field distribution to circular field distribution or vice verse can be freely and easily carried out in any direction of reduction and enlargement or at an equal magnification (mutual conversion from an ellipse to a circle whose diameter is the same as the minor axis or major axis of the ellipse, or vice verse). And, with the field distribution converting optical fiber according to the first aspect of the invention, it is possible to produce excellent field distribution converting optical fibers whose productivity is improved.
Further, the applicant confirmed that, if an inherent spot diameter is made incident in connection with incident light even though the refractive index distribution of an optical fiber in the X and Y directions presented a square-law distribution profile as described above, the field distribution in any one of the X and Y directions does not change when it propagates in the optical fiber. In other words, if the refractive index distribution slope of an optical fiber in the X or Y direction is set to a refractive index slope at which the spot diameter of incident light is not changed, the light field distribution in the direction does not change when light propagates in the optical fiber. In detail, if a condition of w1=w2 is satisfied in an expression of w1xc2x7w2 xcexAg/nxcfx80, it was found that the incident spot diameter does not change.
Therefore, in a field distribution converting optical fiber according to the second aspect of the invention, the refractive index distribution slope of the above core at one side in the X and Y directions was established so that the field distribution in either X or Y direction can meet the condition when light propagates in the optical fiber. Further, a field distribution converting optical fiber according to the second aspect of the invention was constructed so that the optical fiber length is made roughly coincident with a length which is a multiple of the one-fourth cycle length of the field distribution change cycle, in the Z axis direction, of propagating light beams in the XZ plane or YZ plane which may change in line with propagation of light. Thus, the field distribution converting optical fiber according to the second aspect of the invention enables cyclic changes of light field distribution in the lengthwise direction of the optical fiber and mutual conversion from an elliptical field distribution to a circular field distribution or vice versa as in the field distribution converting optical fiber according to the first aspect of the invention.
That is, with a field distribution converting optical fiber according to the second aspect of the invention, since the refractive index profile of the above core at any one of the above X and Y directions of the optical fiber is made into a square-law distribution profile having Ag where the slope becomes w1=w2, it becomes possible to make the cross-sectional profile of the field distribution cyclically circular or elliptical with respect to the propagation distance of optical waves propagating in the optical fiber as in the field distribution converting optical fiber according to the first aspect of the invention, wherein the aspect ratio of the ellipse can be cyclically changed freely.
In addition, with a field distribution converting optical fiber according to the second aspect of the invention, mutual conversion from elliptical field distribution to circular field distribution or vice versa can be freely and easily carried out in any one of the directions of reduction and enlargement of the field distribution or at an equal magnification as in the field distribution converting optical fibers according to the first aspect of the invention. Moreover, the optical fibers can be produced very efficiently.
Further, with a field distribution converting optical fiber according to the third aspect of the invention, in which the cross-sectional profile of a core is made roughly elliptical, roughly ellipsoidal-shaped, or roughly rectangular, it is easy to configure the refractive index profile of the core as described above, and it is possible to make incident the light of an elliptical field distribution profile.
In addition, with field distribution converting optical fibers according to the fourth and fifth aspects of the invention, in which a single mode optical fiber is provided at one end side of the field distribution converting optical fiber according to the first, second, or third aspect of the invention, the optical fibers can be made integral with the single mode optical fiber, wherein, for example, the optical fiber can be directly connected to a laser diode for use.
Further, with a field distribution converting optical fiber according to the sixth or seventh aspect of the invention, the refractive index profiles of the core are, respectively, made into a square-law distribution profile, the slopes of the refractive index distribution in the corresponding X and Y directions are set to equal values, and the optical fiber length is formed so as to be made almost coincident with an odd number times the one-fourth cycle length of the field distribution change cycle, in the Z direction, of a light beam propagating in the XZ plane and YZ plane, wherein by such an optical fiber, it is possible to convert circular field distribution to another circular field distribution, the spot diameters of which are different from each other. In addition, by conversion from circular field distribution to another circular field distribution, the spot diameters of which are different from each other and field distribution conversion from a circle to an ellipse or vice versa by field distribution converting optical fibers according to the first, second, or third aspect of the invention, it is possible to easily connect an optical fiber of elliptical field distribution such as, for example, a laser diode, to a single mode optical fiber.
Furthermore, with a laser diode module according to the invention, laser light of elliptical field distribution, which is emitted from a laser diode, can be converted to a circular profile by a field distribution converting optical fiber and made incident, an optical fiber for optical transmission, which has a circular core, can be optically connected to a laser diode at a low connection loss.